Magnetron sputtering is a well-known method of physical vapor deposition for forming thin films on substrates of various sorts. Sputtering is the technique of choice for depositing many types of films onto to semiconductor waters in the fabrication of semiconductor devices such as integrated circuit "chips." The technique has become particularly widespread in semiconductor fabrication for the deposition of metallization layers of aluminum. Magnetron sputtering is also frequently used to deposit films of other materials onto semiconductor wafers, including, for example, films of titanium nitride, titanium, titanium/tungsten alloy and various precious metals.
As semiconductor device geometries have shrunk, and the density of device components has increased, the demands on sputtering systems have commensurately increased. The typical specifications for sputtered films include such properties as the overall uniformity of the deposited layer, step-coverage (i.e., the ability to cover irregular features on the substrate such as "steps"), and the ability to fill very narrow grooves (trenches) and interconnect vias. The typical specifications for each of these film properties are now substantially greater than they were just a few years ago, and it is expected that this trend will continue as device makers develop new generations of devices. At the same time, the semiconductor wafers used in device fabrication have grown in size, so that wafers having an eight-inch diameter are now common. The increase in wafer size magnifies the difficulty associated with attaining increasingly demanding film specifications across the entire surface of the wafer. For example, achieving film uniformity over an eight-inch wafer is substantially more difficult than achieving the same degree of uniformity over a six-inch wafer.
In magnetron sputtering a plasma is created within a vacuum chamber adjacent to the surface of a sputter target comprising a material to be sputtered. The plasma is formed in a support gas, such as argon, which is introduced at low pressure into the vacuum chamber. (If reactive sputtering is to be performed a reactive gas is also introduced into the chamber.) An electrical potential is created within the vacuum chamber between the sputter target, which typically serves as the cathode of the sputtering system, and an anode. The electrical field causes the support gas to be ionized thereby forming a plasma. A magnetic field is provided to confine the discharge, usually by a magnet system that produces field lines that loop through the surface of the target cathode. The magnetic field traps electrons, increasing the number of collisions between the electrons and the support gas atoms, thereby increasing the ion population and intensifying the plasma.
The positive ions in the plasma are attracted to the sputter target surface which, as noted above, acts as the cathode in the system. Collisions between the positive ions and the surface of the target cause the target material to be ejected from the surface. The ejected atoms travel through the vacuum chamber and a portion of them impinge on the surface of the substrate forming a film.
In order to meet the present and anticipated specifications for sputtered films used in the manufacture of semiconductor devices, a variety of improvements to the basic magnetron sputtering system have been proposed and implemented. These include the use of closed-loop rotating magnet arrays to improve sputtered film uniformity and target utilization, and the use of collimating filters to improve the filling of small diameter vias and narrow grooves.
Closed-loop rotating magnet arrays are used to create a closed-loop magnetic tunnel to confine a plasma which is swept across the face of the sputter target as the magnet is rotated. Examples of such systems are described in U.S. Pat. No. 4,995,958 and U.S. Pat. No. 5,252,194, coassigned herewith, the disclosures of which are incorporated herein by reference. Briefly, the '958 patent teaches how to construct a generally heart-shaped, closed-loop magnet to create an arbitrarily determined erosion profile (for example, uniform erosion), over a large portion of the sputter target, and Ser. No. '251 extends the teachings of the '958 patent to obtain uniform erosion in the central portion of the sputter target. The teachings of these extend to targets that are dish-shaped. For purposes of the present disclosure it is intended that the term "dish-shaped" include planar, convex and concave shapes, or combinations thereof.
An example of a sputtering system comprising a collimation filter is found in U.S. Pat. No. 5,330,628, coassigned herewith, the disclosure of which is incorporated herein by reference. As described in that application, a collimation filter may be used to limit the angles of incidence of sputtered atoms which impinge upon the surface of the substrate. By limiting the angles of incidence it is possible to promote deposition on the bottom and side walls of, for example, a small diameter via. With the apparatus and method taught in the '212 application, sputtering has been successfully used to deposit high quality films into vias having diameters less than 0.5 .mu.m.
Use of a collimation filter requires that the sputter source have highly uniform emission characteristics. Thus, collimation was not a practical technique until a sputter source with suitably uniform emission characteristics was available, such as the sputter source described in the aforementioned patent application Ser. No. 07/471,251. Moreover, collimation requires the use of relatively low pressure sputtering because the scattering that occurs at normal sputtering pressures tends to negate the effects of the filter, i.e., at normal sputtering pressures a substantial number of atoms are scattered after they have passed through the filter, losing the directionality imparted by the collimator.
Aside from the need to operate a collimated sputtering system at low pressure to avoid gas scattering, there are several other advantages of low pressure sputtering as described below.
A typical magnetron sputtering source has a minimum pressure at which a plasma discharge will be initiated, and a lower minimum operating pressure. Nonetheless, as a practical matter for commercial embodiments, the magnetron is always operated above the ignition pressure. This is to protect the system in case the plasma should go out for extraneous reasons. In theory, after a stable plasma discharge is initiated, the operating pressure may be lowered, so long as it does not fall below the minimum operating pressure. However, in a commercial environment, the risk of plasma extinguishment, with the attendant disruption of operation and possible damage to the wafers being processed, is too great.
The inventor has observed that the plasma above a dish-shaped target with a closed-loop magnet behind it tends to spread out as the operating pressure of the sputtering chamber is lowered. The area of the target which contacts the plasma is sometimes referred to as the discharge track, and this discharge track tends to widen as the pressure drops. It is postulated that this occurs as the electrons in the plasma move to higher orbits in the magnetic field. When the edge of the discharge track extends beyond the edge of the target the plasma discharge "goes out." (More precisely, the plasma does not entirely extinguish, but rather it is transformed into a low level Penning discharge. The intensity of this Penning discharge is too low to provide a useful deposition rate.)
From the above observations, it appears that one method of lowering the pressure at which the plasma in a given chamber will extinguish is to simply employ a larger sputter target, such that the edge of the discharge track is able to "spread out" more before it reaches the edge of the target. This is not a very satisfactory solution for at least two reasons. First, employing a larger sputter target will result in less efficient target usage. Since sputter targets can be quite expensive, every effort is normally made to maximize efficiency of use, consistent with the ability to meet sputtered film specifications. Second, the diameter of the sputter target, which is typically wider than the wafer undergoing processing, usually determines the width of the vacuum chamber. For example, in commercial sputtering systems sold by the assignee of the present invention, a target less than twelve inches in diameter is used with eight-inch diameter wafers. Use of a significantly larger target would require that the sputtering chamber be enlarged. This is undesirable for several reasons including added manufacturing expense, greater pump down times (or larger pumps) and larger overall system size.
Several other techniques have been used to reduce the operating pressure of a sputtering system. One such technique is to use a hollow cathode discharge to assist the magnetron discharge. The hollow cathode technique has the added advantage of operating at relatively low voltages. However, in known prior art systems, the hollow cathode is operated within the magnetic field of the magnetron and is coated during system operation. This will eventually lead to particulate problems as flaking off of the hollow cathode occurs, or will dictate frequent hollow cathode cleaning, with a resulting disruption of system usage. In addition, known hollow cathode systems lack cylindrical symmetry, resulting in non-uniform coatings when operating at higher pressures or with larger magnetrons.
Another very new technique is to operate a sputtering source without a support gas at very high power density to achieve self-sustained sputtering. In this technique, the atoms of sputtered material become ionized and sustain the plasma. This technique that appears to be restricted to a limited range of materials, such as silver, gold and copper, which have self-sputtering yields greater than unity. Because of the very high power densities needed to achieve self-sputtering, it is quite difficult to adjust the deposition rate, which is an important process parameter. (It is noted that a support gas at relatively high pressure is still needed to strike the discharge, before going to low pressure operation.)
Yet another technique employs an anti-cathode to achieve low pressure operation. This method prolongs the time that ions stay in the discharge region by using a minimum B magnetic field configuration and electrostatic confinement of ions by means of a virtual cathode. Known embodiments of this type of system result in intense ion bombardment of the substrate, which is undesirable in most semiconductor applications in view of the high risk of damage to devices that have already been formed on the wafer. Moreover, the magnetic geometry used in known anti-cathode systems is likely to result in films which are not highly uniform.
Finally, low pressure sputtering can be accomplished using multipolar magnetic plasma confinement to enhance gas ionization. This technique also may cause unacceptably high ion bombardment of the substrate and is highly sensitive to the relative positions of the magnetron and the multipolar magnets making it difficult to reliably duplicate process results.
Another problem with some of the above solutions to low-pressure sputtering is that the systems do not operate efficiently over a wide range of pressures. While some may function well at very low pressure, they do not work well when the pressure is raised. On the other hand, the pressure at which a film is deposited can influence the properties of the film. Thus, operating pressure can be used as a variable in optimizing a process to create a film with desired properties. It is, therefore, desirable to have a system that can function not only at very low pressure but over a wide range of pressures.
Finally, it is noted that it is generally desirable to operate sputtering systems at the lowest possible impedance to enhance the efficiency of operation.